The history of the muon (g-2) experiments release_rev_67eb7526-19a1-4072-a84d-f64b24281cd4

Abstract

I discuss the history of the muon <jats:inline-formula><jats:alternatives><jats:tex-math>(g-2)</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo stretchy="false" form="prefix">(</mml:mo><mml:mi>g</mml:mi><mml:mo>−</mml:mo><mml:mn>2</mml:mn><mml:mo stretchy="false" form="postfix">)</mml:mo></mml:mrow></mml:math></jats:alternatives></jats:inline-formula> measurements, beginning with the Columbia-Nevis measurement that observed parity violation in muon decay, and also measured the muon <jats:inline-formula><jats:alternatives><jats:tex-math>g</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>g</mml:mi></mml:math></jats:alternatives></jats:inline-formula>-factor for the first time, finding <jats:inline-formula><jats:alternatives><jats:tex-math>g_\mu=2</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>g</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:math></jats:alternatives></jats:inline-formula>. The theoretical (Standard Model) value contains contributions from quantum electrodynamics, the strong interaction through hadronic vacuum polarization and hadronic light-by-light loops, as well as the electroweak contributions from the <jats:inline-formula><jats:alternatives><jats:tex-math>W</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>W</mml:mi></mml:math></jats:alternatives></jats:inline-formula>, <jats:inline-formula><jats:alternatives><jats:tex-math>Z</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math></jats:alternatives></jats:inline-formula> and Higgs bosons. The subsequent experiments, first at Nevis and then with increasing precision at CERN, measured the muon anomaly <jats:inline-formula><jats:alternatives><jats:tex-math>a_\mu = (g_\mu-2)/2</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>a</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mo stretchy="false" form="prefix">(</mml:mo><mml:msub><mml:mi>g</mml:mi><mml:mi>μ</mml:mi></mml:msub><mml:mo>−</mml:mo><mml:mn>2</mml:mn><mml:mo stretchy="false" form="postfix">)</mml:mo><mml:mi>/</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:math></jats:alternatives></jats:inline-formula> down to a precision of 7.3 parts per million (ppm). The Brookhaven National Laboratory experiment E821 increased the precision to 0.54 ppm, and observed for the first time the electroweak contributions. Interestingly, the value of <jats:inline-formula><jats:alternatives><jats:tex-math>a_\mu</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>a</mml:mi><mml:mi>μ</mml:mi></mml:msub></mml:math></jats:alternatives></jats:inline-formula> measured at Brookhaven appears to be larger than the Standard Model value by greater than three standard deviations. A new experiment, Fermilab E989, aims to improve on the precision by a factor of four, to clarify whether this result is a harbinger of new physics entering through loops, or from some experimental, statistical or systematic issue.
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